Hillary Clinton's "no real accomplishments" legacy as secretary of state and new allegations about foreign donations to the Clinton Foundation put the Democrat's presumptive lock on the presidential nomination in jeopardy, former Rhode Island Gov. Lincoln Chafee said Thursday.

"This is all about judgment calls and credibility. I'm not convinced she's going to be the nominee," said Chafee, who's exploring a challenge to Clinton for the 2016 party nod.

If the ex-governor chooses to run, he would be joining Sen. Bernie Sanders, independent from Vermont, who's expected Thursday to announce a bid for president as a Democrat.

"Elections should be about choices," Chafee said. "There are a lot of candidates on the Republican side [and] now we're getting more on the Democrat side."

But Chafee started out as a Republican, serving as a U.S. senator from Rhode Island from 1999 to 2007. In 2010, as an Independent, he was elected governor. Two years into his term, he became a Democrat. He did not seek re-election.

He told CNBC's "Squawk Box" that he'd consider himself left of former senator Clinton on foreign policy.

"We served at the same time in the Senate when we voted on the Iraq War resolution back in 2002. Of course, I voted against it. She voted for it," he said. "The ramifications we live with today are so significant … in the Middle East and North Africa."

Chafee calls the choice to invade Iraq in 2003 "one of the worst decisions in American history."

The Republican-turned-Independent-turned-Democrat said that even though he's switched parties over the years his domestic voting record remained consistent.

Considering himself socially liberal and fiscally conservative, he contended he never changed his principles. "The Republican Party become more about the social issues … and less about balancing the books." That's why he said he left the GOP.

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